Worked on enhancing locale handling stability within the Symfony framework, focusing on both the symfony/http-kernel and symfony/symfony repositories. Addressed edge cases in Accept-Language processing by implementing a default locale fallback mechanism, ensuring the default locale is always considered when no match is found among enabled locales. This solution involved PHP and Symfony back end development, with comprehensive unit and regression tests added to verify correct behavior and prevent future regressions. The improvements increased the reliability and predictability of internationalization features, reducing user-facing failures in multilingual scenarios and strengthening the overall robustness of locale selection across core framework components.